1. Field of the Invention
This invention relates to log periodic antennas and more particularly to log periodic dipole or monopole antennas with foreshortened radiating elements.
2. Description of the Prior Art
U.S. Pat. No. 3,732,572 describes a log periodic antenna in which radiating elements {dipoles} are foreshortened in order to conserve space without adversely affecting antenna performance. This is accomplished by configuring each size-reduced dipole with the interior profile of a double ridged waveguide. By use of this technique, a reduction of 45% in the length of the monopole or dipole antenna elements is achieved. However, when these foreshortened dipoles are used as radiation elements in a practical commonly used log periodic dipole antenna, the width of such antennas cannot be reduced as much. The reason for this is that these antennas have taper angles of approximately 25.degree.. Therefore the available spacing between adjacent dipole elements is relatively small and this prevents the use of foreshortened dipole elements with large width-to-length (B/A) ratios when the planes of these elements are parallel to the antenna axis. For example, the width of a log periodic dipole antenna with a taper angle of 20.degree. can be reduced up to 30%, and one with a 30.degree. taper angle can only be reduced by approximately 25% Log periodic dipole antennas having a very small taper angle (less than 10.degree.), and therefore characterized by long structures and large spacings between adjacent dipole elements, can be reduced up to 40% because foreshortened dipoles with large B/A ratios can be used. Nevertheless the 40% reduction in antenna width is still not sufficient to enable the antenna to fit into the space available in many applications. In addition, log periodic antenna structures with small taper angles may not fit into the space available in the orthogonal direction and a structure with a larger taper angle must be used.
This invention is directed to an improved antenna construction which overcomes these limitations.